Styrene-based resins have been used in wide fields such as for electric or electronic parts, automobile, sundries and other various uses due to their excellent mechanical characteristics, moldability and appearance. However, the styrene-based resins are made from petroleum resources, and recently, their CO2 exhaustion at their production and environmental load at disposal of their wastes have been drawing attention. And materials comprising non-petroleum resources have been desired.
Recently, in view of protecting the global environment, biodegradable polymers capable of being degraded in natural environment by the effect of microorganisms living in soil or water have been drawing attention, and various biodegradable polymers have been developed. Among them, polylactide is of relatively low cost and its melting temperature is high as about 170° C., and it has been expected as a melt-moldable biodegradable polymer. In addition, lactic acid, which is the monomer of polylactide, has become possible to be economically produced by fermentation using a microorganism from biomasses as starting material such as corn, and it becomes possible to produce the polylactide in still lower cost. Accordingly, it is expected not only as a biodegradable polymer, but also as a biomass-based biopolymer, and has been investigated for use also as a polymer of general use. However, on the other hand, it has defects in physical characteristics such as low impact resistance and insufficient softness, and improvement in those characteristics has been desired.
In such a circumstance, as a material of low environmental load, a method is disclosed in which polylactide is mixed with a thermoplastic polymer such as polystyrene, polyethylene, polyethylene terephthalate or polypropylene (patent reference 1). However, when they were mixed in that way, although it was possible to obtain a material of low environmental load, any of them was not sufficient to use as a resin for general use and more improvement of mechanical characteristics were necessary.
On the other hand, a biodegradable resin composition comprising polylactide and an amorphous resin having a glass transition temperature higher than that of polylactide is also known, but further improvement is necessary in view of improving both of heat resistance and impact resistance.
Moreover, an aliphatic polyester resin composition comprising an aliphatic polyester and a polymer of stratified structure (patent reference 3), and a resin composition comprising polylactide polymer and a graft polymer obtained by grafting a vinyl-based monomer to a rubbery polymer (patent reference 4) are also known, but these resin composition do not contain a styrene-based resin and they have defect in heat resistance, and further improvement is necessary to use as a polymer for general use.    [Patent reference 1] JP-A-H06-504799 (page 53)    [Patent reference 2] JP-A-2005-60637 (page 2)    [Patent reference 3] JP-A-2003-286396 (page 2)    [Patent reference 4] JP-A-2004-285258 (page 2)